The purpose of this study is to examine certain epithelial tissues that have the property of actively transporting chloride ions and, which act as osmoregulatory sites at both the systemic and specific organ level. The epithelia used in these investigations are the chloride cell-rich opercular epithelium of the teleost, Fundulus heteroclitus, the rectal gland of the dogfish shark, Squalus acanthias, and the cornea and pigment epithelium of the frog, Rana catesbiana. These epithelia have in common the fact that they actively secrete chloride ions at relatively fast rates and are viable in vitro preparations. The various aspects of chloride transport in these epithelia are studied with a variety of established investigative techniques. These tissue can be mounted in lucite chambers and their transepithelial electrical and ion transporting properties accurately measured. The influences of certain hormones, the effects of specific pharmacological agents, and the influences of other ions on the active chloride transport are observed with these in vitro preparations. Conventional microelectrode techniques are employed to obtain potential profiles and membrane conductances under both control and experimental in vitro conditions. In addition, ion-specific microelectrodes, such as chloride electrodes, are used to measure the intracellular ionic activities in order to determine the distribution of ions across the membranes and to localize the active transport site.